Locking with lockf()

Usage: lockf( fd, command, length );

Command is
F_LOCK -- exclusive lock
F_TLOCK -- nonblocking F_LOCK 
F_ULOCK -- unlock
F_TEST -- check lock status of the file

lockf() is actually a library call, that works via fcntl() system call. fcntl() supports NFS locks when available.

lockf() supports locking only tail ends of a file, fnctl() provides ability to lock arbitrary regions.